1. Field of the Invention
The present invention relates to a nonvolatile semiconductor storage device that is electrically capable of writing, reading, and erasing and a manufacturing method thereof.
2. Description of the Related Art
A market has been expanded for nonvolatile memories, in which data can be electrically rewritten and data can be stored even after the power is turned off. There is a feature in that a nonvolatile memory has a similar structure to that of a MOSFET (Metal Oxide Semiconductor Field effect transistor) and a region capable of accumulating electric charges for a long period of time is provided over a channel formation region. An electric charge accumulating region of the nonvolatile memory is also referred to as a floating gate because it is formed over an insulating layer and insulated with circumference. A control gate is provided over the floating gate through an insulating layer (for example, Patent Document 1 and Patent Document 2: Japanese Published Patent Application No. H5-189984 and Japanese Published Patent Application No. H6-61501).
A nonvolatile memory, of a so-called floating gate type, having such a structure operates for accumulating electric charges in the floating gate by a voltage applied to a control gate and discharging the electric charges. That is to say, the nonvolatile memory of a floating gate type has a mechanism for storing data by taking out and into the electric charges to be held in the floating gate. Specifically, the electric charges are injected into and drawn from the floating gate by application of a high voltage between a semiconductor layer in which a channel formation region is formed and a control gate. As for such a nonvolatile memory, a technique has been developed, in which a semiconductor storage device is formed over a silicon wafer including an insulating layer, a glass substrate, or a plastic substrate as well as being formed into the silicon wafer.
A nonvolatile semiconductor storage device shown in FIG. 29 is disclosed in Patent Document 2. The nonvolatile semiconductor storage device of FIG. 29 has a channel region 2, a source region 7, and a drain region 8 made of a semiconductor layer, a first insulating film 3, a floating gate 4, a second insulating film 5, a control gate 6, a source electrode 15, a drain electrode 11, and a gate electrode 9 over an insulator 1. The source region 7 and the drain region 8 are formed to be in contact with the source electrode 15 and the drain electrode 11 respectively through contact holes formed in an interlayer film 13.